In the blur of competition, often the only thing that a person can hear and understand is his/her own name or (possibly) the name of his/her team (either a school name, or else a special team name like Big Blue, Wildcats, etc.).
So if you're trying to get information across, first get the runner's attention by calling her/his name or, if you don't know that, calling out the team name. It's good to
finish your cheer with the runner's/team's name, as well.
Very brief information is sometimes helpful: "Jim, you're fifth man, Jim!" or "Yorktown, go up and help her [a teammate that's only a few meters ahead], Yorktown!" But again, in the heat of battle the runner may not process much more than a name--and that by itself helps the runner: it's good to be reminded that there are people on your side. So "TJ, attaway, TJ!"--while completely generic--can be a genuine boost.
In general, avoid things like "looking good"--people shouldn't be focused on how they look--and "accentuate the positive" wherever possible. So something like "hold him off!" rather than "don't let that man by!"--you don't want your runner's brain even to entertain the *concept* of letting the man by.
For more specific guidance, it's often a matter of key words that have developed meaning through the course of pracctice--and if you're not involved in practices, it may be hard to say very much! For instance, yelling "relax" at a runner--if s/he hasn't specifically worked on relaxation techniques--may be counterproductive, causing her/him to be self-conscious and tighten up.
Same thing with "arms!": avoid it, unless you know that the runner has drilled a specific response to that. (My teams *had* done specific work for that, so calling "Eric, elbows, Eric!" in the homestraight could actually prompt improved form.) Curiously enough, just calling something like "Maia, great race, Maia!" when the runner is starting the last part of the race can sometimes help trigger a nice kick.
I know other coaches have done actual research into this, so I hope you get some good responses from them.